The taxman is seeking to strike deals with investment banks over long-running offshore tax arrangements in an attempt to recover £2bn in unpaid tax and national insurance contributions.

HM Revenue & Customs has announced the terms of a "settlement offer" for companies with employee benefit trusts (EBTs) as it seeks to resolve hundreds of outstanding investigations. The latest crackdown could result in some investment banks declaring one-off hits to their profits, if they have not made provisions.

The arrangements have been a long-running sore for the government with banks and small companies all employing the schemes. Companies typically pay cash into the trusts free of tax, which would then provide benefits such as loans to employees. HMRC believes it is just a way of paying staff and wants to ensure that income tax and national insurance contributions are paid on the full amount of the benefits received.

Bankers only have to pay tax on notional interest rates of 1%-2% on loans, which do not ultimately have to be repaid. The loans have other tax benefits, including not being part of an individual's estate on death.

Another area of alleged abuse relates to employees' use of assets owned by the trust, including homes or yachts. HMRC believes all such arrangements amount to nothing more than another form of remuneration.

Tax experts said that the settlement offer amounted to timing differences on when the tax due would be payable – meaning penalties or interest due might be forgiven.

Ray McCann, a director at law firm McGrigors, said he thought the terms of the offer were too limited to tempt companies at present.

Jon Terry, a partner at PricewaterhouseCoopers, suggested that the real settlement terms would emerge once the taxman began negotiations with the top investment banks. It is thought that banks and many small entrepreneurial companies have put as much as £5bn into EBTs.

"Most of the investment banks have used EBTs in various guises for 15 to 20 years. People automatically think tax avoidance but you can use them in a plain vanilla way" Terry said.

HMRC is sending letters to companies which are the subject of open EBT inquiries, estimated to be in the low hundreds. But McCann suggested there may be thousands of companies with arrangements that have not received an HMRC inquiry.

Chancellor George Osborne revealed details of new legislation to crack down on EBTs in the budget, in a move the Treasury estimated would raise £750m a year. The settlement discussions relate to structures going back as far as the 1980s.